In mobile communications networks, communications from the eNB to the UEs are referred to downlink communications, and communications from the UEs to the eNB are referred to as uplink communications. The eNB uses a technique referred to a MIMO to increase data throughput in the downlink direction. When using downlink MIMO, the eNB transmits multiple streams of data over the same frequency at the same time by using multiple transmit antennas and digital signal processing techniques such as pre-coding. These multiple streams of data can be targeted to one UE, referred to as single-user MIMO or to multiple UEs referred to as multiple-user MIMO (MU-MIMO). Spatial separation between multiple antennas on the UE or UEs, the characteristics of the transmission path between the eNB and the UE or UEs, called the channel, and pre-coding by the eNB allow the UE or UEs to decode the MIMO signal. In the case of MU-MIMO, each UE will use only the decoded intended for that UE and ignore the other streams.
MIMO can also be used on the uplink channel. Similar to the downlink MIMO technology, the eNB typically instructs a UE to transmit multiple streams of data simultaneously using multiple antennas. The purpose of MIMO in both the uplink and downlink directions is to increase throughput. Although the Third Generation Partnership Project (3GPP) defines uplink MIMO for LTE networks, due to size limits on UEs and cost, LTE UEs rarely support MIMO in the uplink direction, which limits the total uplink data throughput available to all UEs due to this limitation.
A technique, referred to as uplink virtual MIMO, to gain some of the benefits of increased aggregate uplink throughput for multiple UEs in a system is known. When the eNB instructs two or more different UEs to transmit on the uplink channel in overlapping resource blocks, the uplink transmission is referred to as uplink virtual MIMO, because the different UEs together can achieve uplink aggregate throughput similar to those of a multi-antenna MIMO system. Uplink virtual MIMO allows two or more UEs to share the same frequency resources to transmit data. Therefore, although each UE only uses one antenna, multiple UEs transmission forms an equivalent multiple transmit antenna system. Together with multiple receive antennas at eNB side, the UEs and the eNB achieve uplink virtual MIMO. The eNB can decode data from UEs that utilize the same frequency resources at the same time with an advanced receiver that considers the characteristics of the different paths of the spatially separated UEs and achieves higher aggregate UL data throughput.
Before an eNB with uplink virtual MIMO functionality can be deployed in a live network, the uplink virtual MIMO functionality of the eNB should be tested. One way to perform uplink virtual MIMO testing is to use real UEs. However, using real UEs makes the test hard to set up and difficult to control, especially with large numbers of UEs. Accordingly, there is a need for an alternative method to test the uplink virtual MIMO functionality of an air interface device, such as an eNB.